World population growth is well known to exert corresponding pressure on the food supply. As population increases, already costly food ingredients, such as food protein, may become prohibitively expensive for consumption by pets and companion animals. Thus, there is a need for alternative protein sources which do not compete with the human food chain. Such alternative protein sources include typical animal byproducts, such as feathers, hair, fur, wool, bristles, horns, hooves, nails, claws, beaks, outer layer of animal skin, tortoise and turtle shells, whale baleen, porcupine quills and scales, which contain fibrous structural proteins in the keratin family. While keratin protein materials are generally abundant, cheap and sustainable, they also contain relatively high percentages of sulfur-containing amino acids such as cysteine. Cysteine can form disulfide bonds which contribute to the tertiary structure of the keratin protein, making it strong and durable. That structural durability results in low digestibility and makes keratin proteins generally unsuitable in their natural state for use as a source of food grade proteins.
Previous attempts to convert raw keratin-containing material into food grade protein materials have been costly, and resulted in products which were unpalatable and low in protein digestibility. As a result, such keratin-containing raw materials have traditionally been treated as agricultural waste and assigned to disposal or recycling.
Keratin-containing materials can be denatured by subjecting them to harsh physical conditions such as relatively high heat and pressure, for example at 146° C. and 345 kPa for about 30 to 70 minutes. Such treatment can facilitate breakdown of the sulfide bonds, but only incompletely hydrolyzes the keratin. In addition, such conditions are destructive to certain amino acids and may lead to the production of undesirable sulfur-containing non-nutritive amino acids in the end products.
Chemical treatment may also be used to break the disulfide bonds and can generate relatively shorter peptides from keratin. For example, boiling keratin for about 2 to 20 hours at a pH less than or equal to 2.0 to 4.0, or boiling for more than two hours at a highly alkaline pH yielded oligopeptides, polypeptides and free amino acids. However, such harsh treatments may partially or completely destroy certain amino acids, thereby reducing the nutritional aspects of the end product. Alkaline hydrolysis in particular tends to yield undesirable artificial amino acids such as lanthionine and lysinoalanine, the latter of which has been implicated as a renal toxic factor in laboratory rats. Treatment with acidic or basic materials may also produce residual salts in the mixture, which may necessitate additional processing steps for removal.
Thus, hydrolysis of keratin-containing materials by means of harsh treatments such as heat and chemicals suffers from the problems of incomplete hydrolysis and contamination of the food product with undesirable amino acids and residual salts. Moreover, such methods have not been successful in producing highly digestible food products. These methods generally do not yield products having digestibility greater than about 80% as measured by the 2-step method of Boisen and Fernandez (1995).
Accordingly, there is a need for a process that will convert keratin-containing proteinaceous material into a desirable food product ingredient that is nutritious, palatable, and highly digestible by an animal. The desired process should be suitable for use to pretreat protein-containing raw materials, particularly, abundant, sustainable, low cost keratin-containing raw materials, under mild conditions to loosen tightly-packed beta-sheet structure, thereby allowing subsequent enzymatic hydrolysis to efficiently break the peptide bonds of the keratin. The food product should be relatively free of undesirable amino acids and should not require additional processing or minimal processing to remove residual salts, and should also be amenable to industrial processing.